Why did my parts melt?

Conrad Hoffman · May 26, 2021

I made a couple small bars out of 420 stainless, 0.07" x 0.04" x 0.5". Turned out well. I tried to harden them by sealing them up in a hollowed out 303 rod with some paper to kill the oxygen. My logic was that if I got the 303 too hot, I could easily see it on the outside. 303 and 420 should melt at the same temperature. Well, I must have gotten it too hot because one of my parts melted and fused right into the other. Nice clean weld though! The 303 stayed near to perfect on the outside- if I were to shine it up with some Scotchbrite, you'd say it was right off the stock shelf.

Obviously I need to back off on the torch, but why did the 420 melt so easily and the 303 was unaffected even right where it was in the flame?

Trueturning · May 26, 2021

Conrad Hoffman said:
I made a couple small bars out of 420 stainless, 0.07" x 0.04" x 0.5". Turned out well. I tried to harden them by sealing them up in a hollowed out 303 rod with some paper to kill the oxygen. My logic was that if I got the 303 too hot, I could easily see it on the outside. 303 and 420 should melt at the same temperature. Well, I must have gotten it too hot because one of my parts melted and fused right into the other. Nice clean weld though! The 303 stayed near to perfect on the outside- if I were to shine it up with some Scotchbrite, you'd say it was right off the stock shelf.

Obviously I need to back off on the torch, but why did the 420 melt so easily and the 303 was unaffected even right where it was in the flame?

Because the hottest place was inside the contained area like a oven effect? Too small of a containment area. Maybe tumble it so that they move while heat treating possibly. Even heat makes sense as does your material selection since you know what color to heat the material containing the parts then too maybe the 420 just absorbs much more heat. Cooling the outside containment deceiving you about the actual condition of the 420.

Conrad Hoffman · May 26, 2021

I thought about that- the TARDIS effect where the inside is hotter than the outside, but it seems unlikely. Tumbling or rotating it more sounds like a good idea. BTW, my now-melted parts were harder than the hobs of hell, so at least that part worked.

fciron · May 26, 2021

I can certainly see why the parts welded together in an oxygen free environment. Can't see any reason for the inside to be hotter than the outside since the heat source is outside.

Absorbed carbon from the paper lowered the melting point?

Seems unlikely, but if it's not a temperature difference, then it's got to be a chemical difference.

Strostkovy · May 26, 2021

Did they actually melt or was it more of a sintering in a reducing atmosphere?

pavt · May 26, 2021

The thermal conductivity is very different between the two types. 420 has more conductivity, so it will both heat up, and reject heat, faster than the 303. So by the time the 303 is glowing red, whatever is inside of it is glowing white.

303: Thermal Conductivity 16.2 W/m-K
420: Thermal Conductivity 23 W/m.K

When its contained inside the 303, it cannot reject the heat fast enough because there isn't enough thermal difference: the 303 is holding onto the heat at a higher temperature even, maybe.

The 303 would make a good "poor man's" heating element. I would suggest to use a controlled oven to HT the 420, or else use a container that is also made of 420 to continue with the current method because that will give a truer measure of the heat inside.

Greetings from Buffalo.

Conrad Hoffman · May 26, 2021

Now that's a good idea- use a 420 tube for heat treat. I grabbed the 303 mostly thinking it was cheaper and more plentiful, but there's no reason I can't use 420. Probably try that next and still go gentler on the heat. OT, what limits the maximum temperature of IR temp guns? Mine tops out at 550C, so useless for this.

thermite · May 26, 2021

Conrad Hoffman said:
I thought about that- the TARDIS effect where the inside is hotter than the outside, but it seems unlikely. Tumbling or rotating it more sounds like a good idea. BTW, my now-melted parts were harder than the hobs of hell, so at least that part worked.

Why does a dinky little atom bomb - or parts of a lightning bolt - get hotter than the surface of the massive fusion-furnace Sun?

Because the heat cannot escape fast enough vs the rate of conversion or input.

Trapped, as your example was, radiant heat also reflects, crossing the geometric core many times until absorbed.

This can happen to goods in the center of a furnace even if NOT in a container.

Blacksmiths, swordsmiths and steel makers have relied on that sort of concentration effect for millennia - putting the metal down INTO the coals and waiting for it to come up to serious heat off a fire otherwise too small to keep a modest dwelling warm of a winter.

mnl · May 26, 2021

The 303 container is radiating to the outside world which is cold, ~300 K. So some fraction of the heat is leaving the outside of the carrier. If you work out the thermal balance I could see the inside could need to be several degrees hotter to maintain the outside at temperature. If it were in an oven this would not be the case as the walls of the oven would be at about the same temperature as the carrier, but with a torch there will be a fair amount of the area of the carrier that sees the low temperature part of the world.

jim rozen · May 26, 2021

mnl said:
The 303 container is radiating to the outside world which is cold, ...

+1

Another point not yet mentioned: there was no method of temperature control, and not even a way to measure the temperature on even the exterior of the container.

MetalCarnage · May 26, 2021

My 2 cents: It came down to heat dissipation, the outer container was able to dissipate the heat due to it's contact with the air and it's size compared to the small parts inside.

JMHO

-Ron

Strostkovy · May 27, 2021

Where did you point the torch? If they were both in the hot gas from the torch the small parts will heat up far faster than the big chunk of outer metal.

If it was heated solely through the outer tube then they did not get hotter than the outer tube.

Most of the speculation here seems dubious to me.

dgfoster · May 27, 2021

Strostkovy said:
Where did you point the torch? If they were both in the hot gas from the torch the small parts will heat up far faster than the big chunk of outer metal.

If it was heated solely through the outer tube then they did not get hotter than the outer tube.

Most of the speculation here seems dubious to me.

Agreed.

Two thoughts:

Heat flows downhill—-from hotter to cooler always. So the internal temp could not exceed the external temp when the heat source is external. The oven analogies are backwards since those cases involve internal sources.

To weld, steel parts don’t have to melt. They need to get near melting and then can fuse together without flowing. Blacksmiths hammer weld parts without melting them. The hammering helps the process, but only some pressure (gravity alone) can be sufficient. In this case no flux, as commonly needed in atmospheric hammer welding, was needed as the oxygen was depleted inside the tube. A pic would be nice. But I am betting there are no little rivers of steel, just stuck (welded) pieces.

Denis

I did a little looking after posting and found the process I described is more properly called “Forge” welding.

Forge welding - Wikipedia

“Forge welding (FOW) is a solid-state welding process[SUP][1][/SUP] that joins two pieces of metal by heating them to a high temperature and then hammering them together.[SUP][2][/SUP] It may also consist of heating and forcing the metals together with presses or other means, creating enough pressure to cause plastic deformation at the weld surfaces.[SUP][3][/SUP] The process is one of the simplest methods of joining metals and has been used since ancient times. Forge welding is versatile, being able to join a host of similar and dissimilar metals. With the invention of electrical and gas welding methods during the Industrial Revolution, manual forge-welding has been largely replaced, although automated forge-welding is a common manufacturing process.”

I recall seeing bands of iron placed centuries ago around huge columns of cathedrals in Italy to prevent splitting. Those bands perhaps 4” wide and 3/4” thick were forge welded in place. Forge welding examples have been found dating from at least 1000BC.

Conrad Hoffman · May 27, 2021

Here's a photo. Remember, these parts are only about 1/2" long. The one is still in good shape, but the other seems to have melted and fused to it quite nicely. FWIW, I don't believe there's any condition where something fully enclosed can be at a higher temperature than the enclosure. Breaks the laws of thermodynamics... I think. Some chemical explanation makes more sense because there was no damage to the exterior of the "enclosure" whatsoever. I'll be trying another set of parts over he weekend, both 420 and 440C.

pavt · May 27, 2021

Conrad Hoffman said:
Now that's a good idea- use a 420 tube for heat treat. I grabbed the 303 mostly thinking it was cheaper and more plentiful, but there's no reason I can't use 420. Probably try that next and still go gentler on the heat. OT, what limits the maximum temperature of IR temp guns? Mine tops out at 550C, so useless for this.

The limits on temp guns is "How much do you want to spend". Refineries and mills such as my old employer have guns that go up to 1500C and they are not cheap. They are calibrated etc.

Georgineer · May 27, 2021

Conrad Hoffman said:
... FWIW, I don't believe there's any condition where something fully enclosed can be at a higher temperature than the enclosure. Breaks the laws of thermodynamics... I think.

It does, and all the speculation that you can get the inside hotter than the outside is misguided, Baked Alaska nothwithstanding.

https://www.youtube.com/watch?v=Mw-brvKO-Z0

My guess is that although your two different alloys have the same melting temperatures, they have different softening temperatures. This is a well-known property of different tin/lead alloys, particularly plumbers' solder which has a long 'plastic' phase allowing traditional wiped joints to be made.

Supposing the 420 has a lower softening temperature than the 303, and supposing that you exceeded one but not the other, it would make sense for the 420 to slump while the 303 doesn't.

George B.

dian · May 30, 2021

what temp./color did you heat it up to?

Conrad Hoffman · May 30, 2021

A fairly bright orange, aiming for about 1050 C. I'm going by the on-line color charts, but IMO matching a glowing piece of steel to something on the monitor or on paper is a fool's errand, said fool being me. I'm out in the shop cutting and grinding new pieces right now. This time I'm going to experiment with some small scraps, rather than part with hours of work invested. I can learn... if I have to.

cyanidekid · May 30, 2021

Conrad Hoffman said:
Here's a photo. Remember, these parts are only about 1/2" long. The one is still in good shape, but the other seems to have melted and fused to it quite nicely. FWIW, I don't believe there's any condition where something fully enclosed can be at a higher temperature than the enclosure. Breaks the laws of thermodynamics... I think. Some chemical explanation makes more sense because there was no damage to the exterior of the "enclosure" whatsoever. I'll be trying another set of parts over he weekend, both 420 and 440C.

View attachment 321881

303 is a high sulfur alloy to promote free machining. I'd speculate the sulfur is migrating to the 420 and lowering its solidus temperature. sulfur tends to form highly fluid components in a melt, and the effect of the sulfur will be/could be VERY different in a completely different alloy family. there is a big difference between Austenitic and Martensitic "stainless", and the affect of VERY small differences in S content is known to affect welding characteristics even in the "same" alloy.

most anyone who does orbital welding of pipe and tube for food and chem lines would know even PPM quantities can have surprising effects.

(upon reviewing your pic and description, it seems your parts are not welded to the inside of the 303? seems that would happen first in the scenario I postulated....)

magneticanomaly · May 31, 2021

I agree that physics forbods the pieces inside from having been hotter than the container, as long as they were heated through it.
Quick checks of melting points of 420 and 303 SS yield numbers from 2550-2750 deg F....303 if anythinjg a little lower than 420.
But cast iron's melting point is shown as low as 2000-2200, the reduction being caused by the carbon. S o my bet is that the paper intended to absorb oxygen, carburized the pieces inside enough to lower their melting points enough to let them melt while the container did not.

Why did my parts melt?

Diamond

Diamond

Diamond

Stainless

Titanium

Stainless

Diamond

Diamond

Hot Rolled

Diamond

Stainless

Titanium

Diamond

Diamond

Stainless

Stainless

Titanium

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